Forecast the effect of time ticking

Forecast where the values are never changing

This is primarily useful for tests; the forecast range is infinite, but we do still check the precondition, to catch any bugs.

Trivial forecast of values of type () performed by an instance of GetTip .

Specialization of constantForecast .

Compute the upper bound for a range for a forecast across eras.

We have to be very careful here in how we compute the maximum lookahead. As long as we are in a single era, things look like this:

                                         /-------------------\
                                         |                   |
chain     ... - block - block - block [block]                |
                                  |                          v
ledger                           TIP                  VIEW

where TIP is the current ledger tip and VIEW is the last ledger view we can forecast, because the next block [block] to arrive will take effect in the next leger state after VIEW . Note that if the maximum lookahead is zero, this looks like

chain     ... - block - block - block [block]
                                  |      |
ledger                           TIP

where [block] can have immediate changes on the ledger, and so we can't look ahead at all (of course, we always know the current TIP ).

Note that blocks arriving after [block] can only take effect later than [block] , and so they are not relevant for computing the maximum slot number we can compute a ledger view for.

Now, if we are near an era transition, this picture gets a bit more complicated. If the next block is still in this era (that is, unless we are right at the edge), then that imposes one constraint, as before. However, the first block in the next era imposes an additional constraint:

                     ~
                     ~    /------------------\
                     ~    |                  |
         /---------- ~ ---|----------\       |
         |           ~    |          |       |
block [block]        ~ [block']      |       |
  |                  ~               v       v
 TIP                 ~         VIEW
                     ~

There are no restrictions on the relative values of these two maximum lookahead values. This means that it's quite possible for the next era to have a smaller lookahead (to re-iterate, since that era has not yet begun, the first block in that era is at the transition, and so the maximum lookahead applies from the transition point):

                     ~
                     ~    /----------\
                     ~    |          |
         /---------- ~ ---|----------|-------\
         |           ~    |          |       |
block [block]        ~ [block']      |       |
  |                  ~               v       v
 TIP                 ~         VIEW
                     ~

Indeed, if the next era has zero lookahead, when the first block of the next era comes it, it can make changes immediately, and so we can't even know what the view at the transition point is.

Note that if there can be no more blocks in this era, the maximum lookahead of the current era is irrelevant:

      ~
      ~    /----------\
      ~    |          |
      ~    |          |
      ~    |          |
block ~ [block']      |
  |   ~               v
 TIP  ~         VIEW
      ~

We can therefore compute the earliest SlotNo the next block in this era (if any) can make changes to the ledger state, as well as the earliest SlotNo the first block in the next era can; their minimum will serve as an exclusive upper bound for the forecast range.